1. Field of the Invention
The present invention relates to a continuous data server for realizing services of continuous data such as video data and speech data by responding to a plurality of access requests for the continuous data simultaneously, and a method for controlling a continuous data server.
2. Description of the Background Art
A continuous data server apparatus for handling continuous data such as video data and speech data has functions for reading out the continuous data stored in memory devices, and transferring the data continuously in real time while synchronizing in time with terminal devices. Such a continuous data server apparatus is used in fields such as a video-on-demand in which video data for a plurality of movies, etc. are stored and an arbitrary movie is transferred according to a request from a terminal, and an on-line shopping in which information on commercial products is provided in video data through a network.
In such applications, the continuous data server apparatus will receive requests from a plurality of users at random, and therefore the continuous data server apparatus is required to have a capability to transfer respectively different continuous data to many different users simultaneously.
Now, some examples of such a conventional continuous data server apparatus will be described in detail.
An exemplary conventional continuous data server apparatus has a configuration as shown in FIG. 1.
In this conventional continuous data server apparatus of FIG. 1, an access request for the continuous data issued by a user or an application program will be sent through an inter-process communication, a communication via a network, etc. This access request is then notified from one of communication control devices 706 through a bus 720 to a central control device 710, and accepted by the central control device 710.
The central control device 710 then notifies the reading of the requested continuous data to relevant data memory control devices 704, and these data memory control devices 704 read out the specified continuous data from data memory devices 702 and write them into a buffer memory device 708. The central control device 710 then commands the transfer of the data in the buffer memory device 708 to one of the communication control devices 706, and this communication control device 706 transfers the continuous data toward a transfer destination specified in the access request.
The central control device 710 basically comprises a central processing unit (CPU) and a memory device, similarly as an electronic computer. The central control device 710 may be equipped with a communication control function for the purpose of notifying the access request, so that the access request from a user or an application program is notified by the communication control function of the central control device 710 rather than by the communication control device 706.
The data memory devices 702 for storing the continuous data are usually provided in forms of disk devices. For this disk device, a magnetic disk device is used in most cases, but there is also a case of using an optical disk or an opto-magnetic disk device, etc. Apart from the disk device, there is also a case of using a semiconductor memory device such as random access memory (RAM) or electrically erasable programmable read only memory (EEPROM) for the data memory devices 702.
In the conventional continuous data server apparatus of FIG. 1, in order to transfer the continuous data, it is necessary for the central control device 710 to issue control commands to the data memory control devices 704 and the communication control devices 706 through the bus 720 so as to carry out a reading of the continuous data from the data memory devices 702 by the data memory control devices 704 and a transfer of the continuous data to a communication path by the communication control devices 706.
However, in order to realize a transfer of many continuous data, a need to increase the number of data memory control devices 704, data memory devices 702, and communication control devices 706 may arise, and when this need is fulfilled, an amount of communications for the control commands to the data memory control devices 704 and the communication control devices 706 also increases. The central control device 710 is required to issue as many control commands for each data memory control device 704 as the number of data memory control devices 704 each in a size proportional to the number of data memory devices 702 connected to each data memory control device 704, and as many control commands for each communication control device 706 as the number communication control devices 706 in a size proportional to a number of continuous data to be transferred by each communication control device 706, all through the bus 720.
Consequently, there has been a problem of an increasing load on the bus 720 for connecting the central control device 710 with the data memory control devices 704 and the communication control devices 706. In addition, there is a need to issue the control commands for a plurality of times, so that there has also been a problem of delays in control commands for the data memory control devices 704 and the communication control devices 706.
On the other hand, there is another example of a conventional continuous data server apparatus using a configuration as shown in FIG. 2 which utilizes a massively parallel computer in which a plurality of central control devices (CPU) 910 and 911 are connected by a switch (coupling path) 921 with a large data transfer capability (bandwidth).
In this configuration of FIG. 2, it is necessary for the switch 921 connected between the central control devices 910 and 911 to be capable of handling the data transfer between the central control device 910 that has an arbitrary data memory control device 904 and the central control device 911 that has an arbitrary communication control device 906. For this reason, a multi-stage network such as the crossbar switch, the hypercube, or the ATM (Asynchronous Transfer Mode) network will be used for this switch 921.
However, even in this conventional continuous data server apparatus of FIG. 2, the central control devices 910 and 911 are required to issue as many control commands for each data memory control device 904 as the number of data memory control devices 904 each in a size proportional to the number of data memory devices 902 connected to each data memory control device 904, and as many control commands for each communication control device 906 as the number of communication control devices 906 each in a size proportional to the number of continuous data to be transferred by each communication control device 906, all through the switch 921.
Consequently, there has been a problem of an increasing load on the switch 921 for connecting the central control devices 910 and 911 with the data memory control devices 904 and the communication control devices 906. In addition, there is a need to issue the control commands a plurality of times, so that there has also been a problem of delays in control commands for the data memory control devices 904 and the communication control devices 906.
Thus, the conventional continuous data server apparatus has been associated with a problem concerning an increase of a processing load for issuing the control commands from the central control device to the data memory control devices and the communication control devices in proportion to the number of data memory control devices and the number of communication control devices.